The present invention is related to an apparatus for controlling the operation of cantilever-based instruments, and a general method for using the apparatus, using digital electronics except where fundamentally not possible.
Cantilever-based instruments include such instruments as atomic force microscopes, molecular force probe instruments, high-resolution profilometers and chemical or biological sensing probes. An atomic force microscope (AFM) is a device used to produce images of surface topography (and other sample characteristics) based on information obtained from rastering a sharp probe on the end of a cantilever over the surface of the sample. Deflections of the cantilever, or changes in its oscillation, which are detected while rastering correspond to topographical (or other) features of the sample. Deflections or changes in oscillation are typically detected by an optical lever arrangement. A number of other detection means have also been used, including tunneling detection, interferometry, piezo response (strain gauge) and capacitance. In the case of an optical lever arrangement, a light beam is directed onto a cantilever in the same reference frame as the optical lever. The beam reflected from the cantilever is made to illuminate a position sensitive detector (PSD). As the deflection or oscillation of the cantilever changes, the position of the reflected spot on the PSD changes, causing a change in the output from the PSD. Changes in the deflection or oscillation of the cantilever are typically made to trigger a change in the vertical position of the cantilever base relative to the sample, in order to maintain the deflection or oscillation at a constant pre-set value. It is this feedback that generates an AFM image. AFMs can be operated in a number of different imaging modes, including contact mode where the tip of the cantilever is in constant contact with the sample surface, and oscillatory modes where the tip makes no contact or only intermittent contact with the surface. Other information regarding the cantilever can be collected with an optical lever arrangement, including the phase or frequency of oscillation or in-phase and quadrature responses, and this information used to form images of the sample. These images will have a variety of interpretations including sample elasticity, dissipation and adhesive properties. In this manner, it is possible to associate various topographical features with other mechanical, chemical and electrical properties.
A typical prior art optical lever system is illustrated in FIG. 1. In this system a light beam 2, preferably formed by a light source 1 (including a super-luminescent diodes or a laser) with sufficient intensity and lack of pointing or other noise, is directed through a collimation lens or lens assembly 3 and a focusing lens or lens assembly 5 and onto a mirror 6 which directs the focused light beam 7 onto a particular spot on a cantilever 8 in the same reference frame as the optical lever system. The reflected beam 9 is then collected by detection optics, which often include an adjustable mirror 13 and a translation stage for providing an offset to the beam position (not shown), and made to illuminate a position sensitive detector 10 (PSD).
Different AFMs present different schemes for rastering the tip over the sample while detecting cantilever deflection or oscillation and correcting the vertical position of the cantilever base. U.S. Pat. No. Re 34,489, Atomic Force Microscope with Optional Replaceable Fluid Cell, describes an AFM in which the sample is mounted on an arrangement of piezo tube scanners beneath a stationary cantilever. The piezos position the sample in all three dimensions. Another AFM is described in U.S. Pat. No. 5,025,658, Compact Atomic Force Miocroscope. In this AFM, the sample is stationary, lying below an arrangement of piezo tube scanners carrying the cantilever. The piezos position the cantilever in all three dimensions. A third AFM is described in the inventors' co-pending application Ser. No. 10/016,475, Improved Linear Variable Differential Transformers for High Precision Position Measurements. In this AFM, the sample is mounted on a precision stage which employs piezo stacks to position the sample in the x and y dimensions, while the cantilever is mounted on a third piezo stack above the sample which positions it in the z dimension. The x-y position is thus decoupled from the z-position. All three dimensions are sensored with linear variable differential transformers to provide precise positional information. More detailed descriptions of these three AFMs is to be found in the referenced patents and application.
Previously, the electronic circuitry employed to interpret the output from the PSD, calculate the change in the vertical position of the cantilever base relative to the sample required to maintain the deflection or oscillation of the cantilever (the “error signal”) at a constant pre-set value and transmit the signals necessary to effectuate this change, as well as those necessary to form images of the sample, has been analog circuitry or, in relatively recent cases, mixed analog and digital circuitry. Analog and mixed analog/digital circuitry has also been used to detect the phase or frequency of oscillation of the cantilever or in-phase and quadrature responses, where those features have been made available. The repository for the devices implementing this circuitry is typically called a controller, although in some instances, some of the devices have been placed in the computer which serves as an interface between the user and the controller.
The inventors here have proceeded from the position that analog electronics in a controller often contribute noise and other problems in the operation of AFMs and other cantilever-based instruments. The invention disclosed herein, therefore, employs digital electronics in key locations in the controller that lead to improved performance and flexibility. We have also included improved signal routing capabilities based on a mixed analog/digital device that greatly improves the flexibility of the instrument. This new architecture allows all of the functionality of past AFM controllers to be duplicated as well as allowing a great deal of new functionality previously impossible to accomplish with analog circuits.
Analog circuits have used single channel lock-in amplifiers to measure a phase shift between the cantilever and drive signal. FIG. 2 shows a typical such amplifier. Here the AFM is being operated in an oscillatory mode, with the oscillation of the cantilever produced by an oscillator 20, the signal from which is also routed through a phase shifter 21. The phase dependent signal results from a simple analog multiplication of the reference signal from the phase shifter 21 and an automatic gain controlled 22 version of the signal 23 from the PSD (not shown), and low pass filtering 25 the output. The multiplication is performed by an analog mixer or multiplier 24. The output of this type of circuit is dependent on the cantilever phase. To the first order, the measurement is proportional to the cosine of the phase angle. This approach is very simple to implement, but, because of its nonlinearity and limitations inherent in automatic gain control circuits, is very inaccurate for large phase angles.
FIG. 3 depicts another prior art analog signal processing circuit, a two phase lock-in amplifier. In this prior art, the signal 26 (not automatic gain controlled) from the PSD (not shown), is analog multiplied against both a 0 degree reference (the “in phase” component or “I”), and a 90 degree reference (the “quadrature” component or “Q”) and low pass filtering 25 the respective outputs. Each multiplication is performed by an analog mixer (or multiplier) 24. This circuit relies upon a digital device, a direct digital synthesizer 27, for a signal to control oscillation of the cantilever (the oscillation is physically accomplished by a piezo, which is not shown) and the quadrature version of that signal. However, both signals are routed through digital-to-analog converters 28 before the analog multiplication. Similarly, the output from the analog multipliers 24 is routed through another digital device, a digital signal processor 29 (DSP), where the amplitude and phase are calculated from the in-phase and quadrature signals. This too requires converters, in this case analog-to-digital converters 30. In some cases, this DSP is not physically part of the controller, but is instead located on a plug-in card on the computer motherboard. It produces more satisfactory phase results than the single channel lock-in amplifier because it is not subject to the limitations introduced by automatic gain controlled, and the phase in this case is mathematically correct. Nevertheless, analog electronics continue to exact a high price in terms of noise and nonlinearities. The main shortcoming of this approach is that it still relies on analog multipliers. These devices are inherently noisy, nonlinear, subject to frequency and temperature dependent errors. and bleed-through of the mixer references in the output signal.
In addition to the defects and disadvantages already discussed, prior art controllers also have severe upgrade limitations. Typically, they require the purchase of new hardware boxes, cards, modules or some other add-on to alter their functionality or add new features. Even worse, they may require the whole controller be sent back to the factory to do something as trivial as fix a bug in the hardware.